1. Field of the Invention
The present invention relates to a telescopic zoom lens system with a zoom ratio of approximately 2, for use in a single lens reflex camera which has a long back focal distance, and does not change the overall length of the zoom lens system upon both zooming and focusing.
2. Description of the Prior Art
As an example of a zoom lens system in which the overall length thereof does not change upon zooming, there is known, for example, a four-lens-group zoom lens system having either a positive-negative-positive-positive arrangement or a positive-negative-negative-positive arrangement, in this order from the object. With this arrangement, the first and fourth lens groups remain stationary while the second and third lens groups are moved upon zooming.
However, it is difficult to achieve a compact zoom lens system in a four-lens-group arrangement. If the overall length of the zoom lens system is shortened without considering an optical balance among optical elements therein, the optical power (hereinafter, power) of each lens group has to be made stronger, so that the correcting of aberrations becomes difficult.
Furthermore, the entire first lens group is generally used for focusing. However, since the entire first lens group has a large diameter and the weight thereof is heavy, such a zoom lens system is not suitable for an autofocus (AF) camera. Moreover, if the entire first lens group constitutes a focusing lens group, spherical aberration, at a closer distance, tends to occur in the negative direction.
The present invention provides a zoom lens system for use in a single lens reflex camera which does not change the overall length of the zoom lens system upon both zooming and focusing, and which is compact and has a high optical performance.
According to an aspect of the present invention, there is provided a zoom lens system including a positive first lens group, a negative second lens group, and a positive third lens group, in this order from the object. Upon zooming from the short focal length extremity to the long focal length extremity, the positive first lens group remains stationary, the negative second lens group moves toward the image, and the positive third lens group moves toward the object. The positive first lens group includes a first sub-lens group which is stationary and a second sub-lens group which functions as a focusing lens group.
Due to the above arrangement, the positive first lens group is made stationary when zooming is performed; and focusing is carried out by moving the second sub-lens group which is positioned behind the first sub-lens group. Consequently, the zoom lens system does not change the overall length thereof upon both zooming and focusing, and aberrations which occur at a close-distance photographing can be reduced. Furthermore, the zoom lens system is suitable for an AF operation, since the weight of the focusing lens group can be made lighter.
The zoom lens system preferably satisfies the following conditions:
1 less than |m2W|xe2x80x83xe2x80x83(1) 
1 less than |m3W|xe2x80x83xe2x80x83(2) 
1 less than Z2/Z3 less than 2 xe2x80x83xe2x80x83(3) 
wherein
m2W designates the lateral magnification of the negative second lens group at the short focal length extremity;
m3W designates the lateral magnification of the positive third lens group at the short focal length extremity;
Z2 designates the ratio (m2T/m2W) of the lateral magnification (m2T) of the negative second lens group at the long focal length extremity to the lateral magnification (m2W) of the negative second lens group at the short focal length extremity; and
Z3 designates the ratio (m3T/m3W) of the lateral magnification (m3T) of the positive third lens group at the long focal length extremity to the lateral magnification (m3W) of the positive third lens group at the short focal length extremity.
The zoom lens system can satisfy the following conditions:
1.0 less than fw/f1 less than 2.0xe2x80x83xe2x80x83(4) 
0.5 less than fw/f1a less than 1.0xe2x80x83xe2x80x83(5) 
wherein
fw designates the focal length of the entire the zoom lens system at the short focal length extremity;
f1 designates the focal length of the positive first lens group; and
f1a designates the focal length of the first sub-lens group.
The zoom lens system preferably satisfies the following condition:
0.05 less than L1a/f1 less than 0.15xe2x80x83xe2x80x83(6) 
wherein
L1a designates the distance between the first sub-lens group and the second sub-lens group when an object at an infinite distance is in an in-focus state.
The first sub-lens group can include a negative lens element and a positive lens element, in this order from the object. The second sub-lens group can include a negative lens element and a positive lens element, in this order from the object.
Each of the two positive lens elements in the positive first lens group is preferably made of a glass material having the Abbe number of more than 75.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2002-133452 (filed on May 9, 2002) which is expressly incorporated herein in its entirety.